Methods and apparatus pertinent to the instant invention are disclosed in the following U.S. patents:
U.S. Pat. No. 2,769,918 issued to Tittle on Nov. 6, 1956, which discloses apparatus having a neutron source and a single epithermal neutron detector to be used for the purpose of determining the hydrogen content of an adjacent earth formation;
U.S. Pat. No. 2,776,378 issued to Youmans on Jan. 1, 1957, which discloses a source of fast neutrons and two detectors for simultaneously measuring the thermal neutron flux and the gamma ray flux at two different points to compensate for the effect of hydrogen in an adjacent formation and to compensate for the effects of variations in the average capture cross section of the material in the formation;
U.S. Pat. No. 3,321,625 issued to Wahl on May 23, 1967, which discloses a gamma ray source and two gamma ray detectors to determine the density of the surrounding earth formation while compensating for the drilling mud filter cake masking the earth formation adjacent to a well bore;
U.S. Pat. No. 3,483,376 issued to Locke et al., on Dec. 9, 1969, which discloses a fast neutron source and two adjacent thermal neutron detectors to determine the porosity of the surrounding earth formation which may be shielded behind well casing and cement in a well bore;
U.S. Pat. No. 4,004,147 issued to Allen on Jan. 18, 1977, which discloses a source of fast neutrons and a pair of thermal neutron detectors spaced apart from said source to determine the porosity of an earth formation surrounding an uncased well bore.
Another reference which also furnishes some valuable background is a paper by L. S. Allen et al., entitled "Dual-Spaced Neutron Logging for Porosity" which was published in the February, 1967, issue of GEOPHYSICS.
As is evident upon perusal of the above-stated references, much work has been performed toward determining the porosity and density of earth formation employing nuclear logging techniques.
With regard to the instant invention, the most pertinent of the above-referenced materials is considered to be U.S. Pat. No. 3,321,625 issued to Wahl. Wahl discloses the detection of gamma rays at two positions spaced apart from a gamma ray source to determine the density of earth formation surrounding an uncased well bore wherein the "mudcake" effect is compensated for. The Wahl method (commonly identified as the gamma-gamma density method) and tool is not considered by applicant to be suitable for determining the porosity of earth formations adjacent to a well which are obscured by the shielding effects of the well casing and cement surrounding a typical cased borehole.
To the best knowledge of the Applicant, the instant invention is the first and only fast neutron technique suitable for providing quantitative values of porosity of earth formations located behind the casing and cement of cased wells.
This invention can fulfill several needs in a manner not previously available. The invention may be used in production logging of cased wells to re-evaluate existing reservoirs. The invention may also be used to monitor water injection or gas injection intervals in cased well bores. The invention may also be used for logging cased well bores in routine wells where only restricted well logging is authorized for the sake of economy. Further, the invention may be used for monitoring subsistence wells in depleted zones, storage zones, and the like. Still further, the invention may be used in the reworking of old wells which have never been logged or which previously have had uncased hole logs only (such as resistitivity logs), particularly in large old fields. The instant invention may be used in lieu of or as a supplement to substantially all applications where the above-mentioned gamma-gamma density log is currently being employed.
A primary object of the instant invention, which fulfills a long recognized need, is to measure the porosity of earth formation shielded and obscured behind well casing and cement conveniently and effectively, and to provide quantitative values of such porosity.
Another object of the instant invention is to measure the porosity of earth formations adjoining uncased boreholes with the same well logging tool.